A short History of the Real-time ultrasound scanner

Dr. Joseph Woo

The innovation which had soon completely changed the practice of ultrasound scanning was the advent of the Real-time scanners. The first real-time scanner, better known as fast B-scanners at that time, was developed by Walter Krause and Richard Soldner (with J Paetzold and and Otto Kresse) and manufactured as the Vidoson® by Siemens Medical Systems of Germany in 1965. D Hofmann, H Holländer and P Weiser published it's first use in Obstetrics and Gynecology in 1966 in the German language. Hofmann and Holländer's paper in 1968 on "Intrauterine diagnosis of hydrops fetus universalis using ultrasound" also in German, is probably the first paper in the medical literature describing formally the diagnosis of a fetal malformation using ultrasound.

The Vidoson used 3 rotating transducers housed in front of a parabolic mirror in a water coupling system and produced 15 images per second. The image was made up of 120 lines and basic gray-scaling was present. The use of fixed focus large face transducers produced a narrow beam to ensure good resolutions and image. Fetal life and motions could clearly be demonstrated.

The Vidoson*, its working mechanism and the resultant image of a fetal face and hand.
The transducer housing is mounted on a mobile gantry and rigidly connected to the main console.
The scanning frequency was 2.25 MHz. Scaling and caliper functions were not present.

Malte Hinselmann, using the Vidoson, demonstrated in 1969 the universal visualization of fetal cardiac action from 12 weeks onwards. The Vidoson was popular in the ensuing 10 years or so and were used in many scientific work published from centers in Belgium, Italy, Germany, Vienna and North America. The initial popularity was not based on its image resolution but rather its ability to allow the operator to display and study movements, such as fetal cardiac motion, gross body movements and fetal breathing movements (see also Part 3). In the International Symposium on Real-time ultrasound in Perinatal Medicine held in Charleroi, Belgium in 1978, most of the presentations were based on results from the Vidoson.

" ......... For almost ten years, real-time ultrasound has been used in many obstetrics departments. By means of an apparatus which has since become technologically outdated many doctors, technicians and expectant mothers had, at the time, the moving experience of being able to observe the living fetus. This seems to me to have been a psychological break-through. For the first time, the human eye pierced the 'black box' of the womb...... Those who were present in obstetrics departments when this technique was first used soon realized how indispensable it was proving to be in providing a valid means of observation of the fetus and its health, in ascertaining its age and studying its morphology and growth........ . Over the last three years, the appearance of the multitransducer scanner has brought about substantial technical progress. At the same time, but quite independently of this, numerous studies on fetal breathing movements, fetal behavior and neonatal cardiology were published ....... ." --- R. Chef, Maternité Reine-Astrid, Charleroi (Belgium), in the foreword to the Proceedings of the International Symposium on Real-time Ultrasound in Perinatal Medicine held in 1978 at Charleroi.

James Griffith and Walter Henry produced a mechanical oscillating real-time scanning apparatus in 1973 which was capable of producing clear 30 degree sectoral real-time images of good resolution. The transducer was employed extensively in cardiac scanners and the design was hailed as one of the significant milestones in the development of echocardiography. Other mechanical systems published included an oscillating design with membrane-oil coupling from W N McDicken in Edinburgh (1974, produced commercially as the EMI® Emisonic 4260), a continuously rotating wheel with radially-mounted transducers from Hans Hendrik Holm in Denmark (1975), and a single transducer direct-contact design from Reginald Eggleton in Indiana (1975). Toshiba®, in Japan produced their first prototype real-time mechanical sector scanner in 1975, the SSL-51H. A number of others were available commercially soon afterwards and sold well such as the circular rotating system Combison 100 from Kretztechnik® of Austria (1977), produced under the ingenuity of Carl Kretz.

Although these have relatively heavy probes they produced outstanding real time resolution in the near and far field (because of highly focused beams resulting from the relatively large curvatured transducers and the lens apparatus) and with much less image-degrading electronic noise that was associated with electronic scanners that soon became available at around the same time.

The large hand-held circular rotating transducer from KretzTechnik® and the resultant sector image.
The transducer is connected to the main console by a flexible cable.

The concept of the multi-element linear electronic arrays was first described by Werner Buschmann in an ophthalmologic application in 1964 in East Berlin. His probe consisted of 10 small transducers mounted on an arc-shaped appartus to fit over the eye. A number of designs followed on the same concept. Jean Perilhou and her group in France, working under the auspices of the Philips® Company, described a multi-element scanning array in 1967, although they do not produce images in a real-time fashion. The real-time array concept was further expanded by Nicolaas Bom at the Thoraxcenter, University Hospital, Erasmus University in Rotterdam, the Netherlands. His initial design in 1971, which was described in his application for a Dutch patent, consisted of only 20 crystals (each 4mm x 10mm). The probe face was 66 mm long and 10 mm wide and produced 20 scan lines. It operated at a frequency of 3.0 MHz sweeping at a frame rate of 150 frames/sec. The axial resolution was 1.25 mm while the beam width at 6 cm was 10 mm. This abeit simple and inadequate design at that time has evolved into the very sophisticated real-time scanners that are widely available today.

In collaboration with cardiologist Paul Hugenholtz and local Dutch company Organon Teknika, they produced in 1972 the "Multiscan system", notably the earliest commercial linear array scanner in the world, mainly aimed at cardiac investigations.

The transducers operated at either 2.25 or 4.5 MHz, again with 20 crystals producing 20 scan lines. The lateral resolution of this improved version at a dynamic range of 10dB was 3.7 mm at 6 cm and 6 mm at 10 cm depth. It did not sell very well though because of its relatively primitive resolution and its inability to image abdominal structures adequately.

In Japan, Rokuro Uchida at Aloka® (see also Part 1) had similar research on the array technology in the late 1960s predating their European counterpart. In 1971 they published in Japanese (and presented at the Japan Society of Ultrasonics in Medicine) a system based on 200 closely interspaced transducers. Electronic switching and use of overlapping groups of 20 small elements yielded 2-D images with a field depth of 20 cm at a rate of 17/frames per second. The company produced their first prototype linear array scanner in the same year. The model however, was not produced commercially or given a model number. The first commercial linear array scanner from Aloka® only debuted in 1976. Toshiba® produced their first commercial real-time linear array counterpart in the same year, the SSL-53H, aimed at abdominal applications. Like the Aloka® this was a huge machine considering present day fabrication standards.

Subsequent to Bom's work and the research in Rotterdam, Leandre Pourcelot in Tours, France was experimenting with more advanced segmented sequence transducer-array scanning possibilities in 1972, in order to enhance the lateral resolution of the devices. Donald L King at the Columbia University in New York described a 24-elements segmented sequence linear-array cardiac scanner in 1973, in collaboration with the Hoffrel Instruments Inc.&reg, at Norwalk, Connecticut. In his design 3 crystal elements were fired simultaneously to produce a single pulse of ultrasound. The echoes returned from the 3 reflections were written into a single line on the scan. The crystals were stepped in a "1,2,3,... 2,3,4,... 3,4,5... " manner. There was however no delay lines for the implementation of 'focusing' techniques.

Also concurrent to King's work was the work in from the Tony Whittingham group at Newcastle-upon-tyne in England, where crystal stepping techniques were also being investigated.

"...... I tried various ideas, but the one that worked was to make an array of very narrow rectangular elements and to use a group of these to form a square aperture. This group of elements defining a composite transducer would scan the line in front of itself. Then you drop one element off from one end, put another element on at the other end, infront of the group, and advance the active group along the array in this way. When I was doing this I was totally unaware that it would work. I hope it would work, but I was worried that there would be cross-coupling from the end elements of the group into what should have been passive elements, so that you might not be able to get a well-defined active aperture. But it did work, and that proved to be the way forward, because you could make finer and finer elements and get more lines into the array ........" -- Tony Whittingham, describing his work in real-time imaging in the mid 1970s. ^

Images of the earlier models were nevertheless hampered by the problems of small crystal size, lobe artifacts, unwanted specular reflections, low dynamic range, unsatisfactory lateral resolution and image noise from electronic processing. There was an overwhelming need for the refinement of beam characteristics. Fredrick (Fritz) Thurstone, Olaf von Ramm and H Melton Jr at the Duke University published some of the earliest and most important work on electronic focusing using annular arrays ('71-'74), both on transmit and receive. Similar techniques were subsequently employed in the design of linear arrays transducers. Basing on these designs, a number of centers and private laboratories were starting to embark on making machines geared to examination of the abdomen. Albert Macovski at the Standford Universityfiled a patent in 1974 for a circular array where the elements generate dynamically focused beams that could also be swept through space by adjusting the delays to the array. This was one of the more advanced designs in dynamic focusing techniques. Another important design - "signal processor for ultrasonic imaging" was described by William Beaver of the Varian Associates® in Palo Alto in California in 1975 where selection of scan angles and focusing distances were effected. George Kossoff in Australia also filed a patent in 1973 on a linear array system incoporating phased-focusing electronics.

It was Martin H Wilcox, founder and engineer at the Advanced Diagnostic Research Coporation (ADR®, a company founded in 1972 in Tempe, Arizona) who designed and produced one of the earliest commercially available models of a linear-array real-time scanner in 1973 and set the standard for subsequent designs to follow. The array contained 64 crystals in a row (3 times the number in the earlier cardiac counterparts and 3 times as long and wide), fabricated with the best material available and in the best accoustic configurations and using 'stepping' crystals techniques. This was the first 'good-resolution' abdominal linear-array scanner that was in the commercial market.

Their second model the 2130 marketed in 1975 had brought the linear-array principle and the application of 'focusing techniques' to commercial fruition. It was a big hit in the United States and had sold over 5000 units worldwide. In 1980 they marketed a new 3.0 MHz variable focus transducer to be added on to the 2130. The new transducer contained 506 crystal elements, boasted both mechanical and phased focusing, improved gain and reduced noise, much quieter transducer operation, and switchable focal zones. The image had twice the number of data lines and probably the best real-time resolution in the industry at that time.

" ....... In Dallas, Texas, Ian was shown the first real-time scanning machine brought from Phoenix, Arizona, by some talented young men. Ian was of course, wildly excited. They wanted to carry him off to Phoenix to show him more, but sadly Ian couldn't change his next commitments. However, it wasn't too long before he had one of his own. ......."
--- Alix Donald, wife of the late Ian Donald, speaking in 1998 about their first encounter with the ADR real-time scanner in the early 1970s. ref.

ADR® merged with ATL® (Advanced Technology laboratories®, see below) in 1984. ADR® produced the 2150 in 1980 and the last model under the ADR label, the ADR 4000 in 1982.

Linear array and annular array technology had also been heavily investigated by the Japanese since the early 1970's who had been moving ahead very successfully with innovative electronic engineering in many domestic, commercial and professional sectors. Commercial linear array models from companies like Hitachi®, Toshiba®, and Aloka® soon began to dominate the world market. Hitachi® produced their first linear array scanner the EUB-10 in 1976, followed by the EUB-20 in 1977 and the EUB-22 in 1979. The EUB-20Z produced in 1978 already incorporated the world's first digital scan converter.

The Toshiba® SAL-10A and the more portable SAL-20A (pictured on the left) and SAL-30A, which were marketed in 1977, 1978 and 1979 respectively, and the Aloka® SSD-202 (1979), SSD-203 (1980), SSD-240 (1981) and SSD-256 (1982) were popular and had found their way into notable Institutions outside of Japan such as the King's College Hospital in London (Campbell), the Herlev (Gentofte) Hospital in Copenhagen (Holm), the Hospital Universitaire Brugman in Belgium (Levi) and were employed in many important early studies. The SAL-10A which was designed by acclaimed Japanese engineer Kazuhiro Iinuma, received many commendations. Other popular early choices included the Axiscan 5 (1976) and Abdoscan 5 (1979) from Roche Kontron®, the Sono R from Philips® (1978), the RA-1 (1980) and the Imager 2300 (1981) from Siemens®, and the LS 1500 (1981) from Picker/ Hitachi®. Aloka® scanners were marketed in the United States under the brand Narco Air-Shield®. Diagnostic Sonar® Ltd, a company founded in 1975 in Edinburgh, Scotland produced the first electronic real-time scanner, the System 85 in the United Kingdom in 1976.

Early scanner probe was bulky to fit on the abdomen ***
Images from early real-time scanners had obstrusive scan lines, low dynamic range and resolution.

Many of the early models typically had very large probes housing an array of some 64 transducer (crystal) elements arranged in a linear row, and operating with sequential electronic switching or dynamic focusing. It was not until the early 1980s that probe size had gotten smaller and image resolution improved. The latter was acheived largely through an increase in the number of transducer crystals (or channels, from 64 to 128), improvements in transducer crystal technology (going into broad-band and high dynamic range), increasing array aperture (more crystals firing in a single time-frame), faster computational capabilities, improving technical agorithms for focusing on receive (increasing the number of focal zones along the beam), incoporating automatic time-gain controls and progressively replacing analog portions of the signal path to digital.

The rapid reduction in the physical size of the machine console in the later half of the 1970s ( See Aloka and Toshiba's early products above) was the direct result of the invention of the microprocessor and the evolution of the minicomputer into the microcomputer.

By the late 1960's, computers built from discrete transistors and simple integrated circuits (IC) already existed. The first practical IC was fabricated in 1959 at Fairchild and Texas Instruments and Fairchild began its commercial manufacture in 1961. As manufacturing technology evolved, more and more transistors were put on single silicon chips with the maximum number of transistors per chip doubling every year between 1961 and 1971. They progressively became a device containing many circuits and was called a LSI (Large Scale Integration). The PDP-11 minicomputer from DEC® containing many ICs and LSIs, was used in many scanner consoles up to the late 1970s. The UNIBUS architecture used by the CPU in the PDP-11 was particularly suited to communicating with memory and peripherals.

Towards the end of 1969 the structure of the smaller programmable calculator had emerged. Intel®, under contract from Japanese company Busicom for the design of a small desktop programmable calculator, produced the world's first microprocessor chip the 4004 in 1971. In order to create a chip of such complexity, new semiconductor design technologies had to be developed. The 4004 is considered the first general-purpose programmable microprocessor, even though it was only a 4-bit device. The original 4004 measured 1/8th of an inch long by 1/16th of an inch wide and contained 2,300 transistors. It ran at 108 KHz and executing 60,000 operations in a single second. It had about the same amount of computing power as the original ENIAC which weighed 30 tons, occupied 3,000 cubic feet of space and used 18,000 vacuum tubes. Today's 64-bit microprocessors are still based on similar designs, with more than 8.5 million transistors performing hundreds of millions of calculations each second.

The 4004 was followed quickly by the 8-bit 8008 microprocessor. The 8008, which contained 3300 transistors, was originally intended for a CRT application and was developed concurrently with the 4004. By using some of the production techniques developed for the 4004, Intel® was able to manufacture the 8008 as early as March 1972. The 8-bit era between about 1975 and 1980 had a major impact on household computing and industry because the first few microprocessors were available at very affordable prices.

The 8008 microprocessor from Intel® was however relatively crude and unsophisticated. It had a poorly implemented interrupt mechanism and multiplexed address and data buses. The first really popular general-purpose 8-bit microprocessor was Intel's 8080, in production in early 1974. This had a separate 8-bit data bus and 16-bit address bus. It ran at 2 MHz with 6000 transistors. It has essentially ten times the performance of the 8008.

Shortly after the 8080 went into production, Motorola® created its own competitor, the 8-bit 6800, containing 4000 transistors and destined for use in automotive and industrial applications . Although the 8080 and 6800 were broadly similar in terms of performance, they had rather different architectures. The 6800 was, to some extent, modeled on the PDP-11 and had much a cleaner architecture than the 8080. Other newer processors followed and found their way into industrial operations including medical scanners and equipments.

At around the same time, steered-beam phased array transducers and annular array transducers with more complicated electronic circuitry were described, and had found their way into echocardiographic examinations because of the relatively small contact surface.

The phased-array scanning mechanism was first described by Jan C Somer at the University of Limberg in the Netherlands and in use from 1968, way ahead of its time and several years before the appearance of linear-arrays systems. The principle of phased-arrays had probably been known much earlier where the technique was engaged in underwater submarine warfare and hence the technology was kept confidential. Fredrick (Fritz) Thurstone and Olaf von Ramm at the Duke University published one of the earliest and most significant phased-array designs in 1976, which was incorporated into a number of commercial sector-array scanners. Other early significant contributors to the beam-former techniques included Albert Macovski at Standford University and Samuel Maslak at Hewlett Packard®. Maslak later founded the Acuson Corporation (see also Part 3).

The Kossoff group in Australia had also made significant progress in the annular phased array transducer designs as early as 1973 and the technology was incoporated into their water-bath scanner, the UI Octoson. In England, EMI® produced the Emisonic 4500, a phased-array sector scanner which was nevertheless expensive, electronically noisy and had inferior resolution in the near fields. Early phased-arrays in the late 70s were all used in cardiac applications. Important manufacturers included Varian® and Irex®. In the first half of the 1980s, image quality in phased-arrays had continued to improve and some outstanding designs had come from Irex® and later on Elscint® (Dynex) and Hewlett Packard®. Despite the small probe size, phased-array sector scanners had never been popular with Obstetrical and Gynecological examinations.

Compound static scanners continued it's tradition of being very huge bulky machines (probably influenced by the design norms of other imaging modalities such as tomographic x-ray machines). New static scanners which were in great demand and produced excellent images were still on the drawing board and production line in the early 1980s. It was believed that real-time scanners would play only a complimentary role to static scanners in the assessment of moving structures. These static machines however were starting to be replaced or phased out at a rate that was faster than expected. There was apparently little practical, economical or clinical advantage of these costly machines over the more mobile and flexible electronic real-time scanners.

There were initially many who were so used to and skillful at operating the static machines that they were unhappy to switch over entirely to the real-time counterparts. They were also anxious about the latter's limited field of view, poorer resolution and allegedly 'less accurate' on-screen measuring system that they have only started to get used to not too long ago. Static scanners were not completely out of the scene until about 1985-86. The switch-over had serious financial implications to some companies who had a large inventory of static scanners.

Scanner engineering itself was soon in the hands of commercial companies rather than clinical personnel as advanced computer technologies were fiercely incoporated into each design to manipulate beam characteristics and signal processing to produce the best possible scan images. Important early manufacturers of real-time equipments included Aloka®, Hitachi®, Toshiba® and Shimadsu® from Japan; EMI®, Diagnostic Sonar®, Siemens®, KretzTechnik®, Bruel and Kjaer®, GEC®, Philips®, Rohe® and Roche-Kontron® from Europe and ADR®, Diasonics®, Dynex®, Ecoscan®, Elscint®, Hewlett-Packard®, Irex®, SKI®, Phosonic Searle®, Technicare® (acquired Unirad®) and Xonics® from the United States. The application of ultrasound in Obstetrics and Gynecology had since then undergone an explosive proliferation all over the world. By the early 1980s there were over 45 large and small ultrasonic scanner manufacturers worldwide.

It was somewhat regretable to see that British manufacturers has failed to keep up with developments made by other leaders in array technology, notably those from the United States and Japan. This was probably reflection of a similar trend in other arenas of electronic and micro-processor development in these countries. It is also of interest to note that the Siemens Vidoson® and the Octoson®from Australia both did not sell in North America at all. Both had the disadvantage of being cumbersome when scanners from other manufacturers were rapidly getting better in resolution and manuevability.

Because of its smaller convex contact surface, the curvilinear or convex sector-array fits much better on the abdomen and allows for a wider field of view than does the linear-array configuration. Work on the fabrication of an electronic convex array had started in the late 1970s in the larger Japanese companies such as Hitachi® (publishing their convex attachment to their EUB-10A scanner in 1978; Aloka® (filing their patent on the convex scanner in 1980), as well as in U. S. companies notably the North American Philips® and the Picker Corporation®, who had filed their patents for convex arrays and processors in 1979 and 1980 respectively. The first commercially available convex array transducer apparently only debuted in 1983 in a scanner from Kontron Instruments® in Europe, the Sigma 20, which was designed especially for use in Obstetrics and Gynecology. Hitachi® in Japan marketed their model EUB-40 with their new convex array later on in the same year.

Toshiba® introduced a similar array in 1985, in their new scanner model SAL-77A. Interestingly, the design actually replaced an earlier model (by only about 9 months) the SAL-90A which boasted a new "trapezoid" linear array in which the face of the transducer was flat but a trapezoid-shaped image was produced from the 128 transducer elements using phased electronics. Surprisingly American machines in 1985 were still using linear arrays, which in time were totally replaced by the new convex configuration.

Model number of some of the scanners made after 1980 from important manufacturers are listed here with the year in which they were marketed. Also view pictures of some of these scanners.

Skin Coupling material for ultrasonic transmission has also switched from oil to a water-soluble (non cloth-staining) gel medium. One of the more well-known manufacturers was the Parker Laboratories® at New Jersey. Images are commonly recorded on "peel-apart" Polaroid® films (the Type 611 was most commonly used) or multi-format radiographic films (6-9 images on one film) using dedicated video imagers.

Worthy of mention here was the attempt in the late 70s and early 80's to miniaturize scanners so that they could be portable and be used at the bedside. Three such examples were the all-in-one MiniVisor® from Organon Teknika® in the Netherlands, the Bion PSI-4000 from Bion Coporation® in Denver and the Shimasonic SDL-30 from Shimadzu Corporation®, Japan. The Minivisor® (available from 1979) was a spin-off from Bom's laboratory. It was battery operated, shaped like a mushroom, had no wires and used a 2-inches display with an on-screen caliper system and digital readout. The transducer is fused to the bottom of the device similar to a 'large' fetal pulse detector. Juiry Wladimiroff suggested in 1980 the device would be useful for routine BPD screening. Nevertheless the popularity of these machines were short-lived for several important reasons pertaining at that time: The resolution was unsatisfactory because of the available electronics. The images of the 'standard' and larger devices, as well as their overall 'portability' have seen rapid improvements round about the same time; and thirdly, real-time ultrasound has very rapidly established itself as a definitive diagnostic entity and the concern for good image information appeared to overide that of the extra portability.

The invention of the real-time scanner had enabled much more effective diagnosis of many fetal malformations and in particular cardiac anomalies which hitherto was impossible to diagnose accurately. (see Part 3). Fetal sonography and prenatal diagnosis (a term which was only coined in the 1970s) had emerged as the 'new' science in Obstetrics and fetal medicine.

In the early 1960s, using a proprietary A-mode vaginal scanner from KretzTechnik, Alfred Kratochwil in Austria had reported on fetal heart pulsation at slightly over 6 weeks menstrual age. He also developed a thimble attachment transducer to facilitate vaginal sonography with pelvic examination. For obviously reasons, A-mode vaginal scanners had completely disappeared after the advent of articulated-arm B-mode equipments. Articulated-arm ' vaginal' scanners were considered operationally objectionable and has never been created. Athough the need and technology were there, the real-time transvaginal, or endovaginal transducer was not "invented" until 1985 when KretzTechnik® of Austria produced their firstreal-time mechanical vaginal sector scanner. The transducer had a scan angle of 240 degrees and was designed with the use for transvaginal ovum retrieval in mind, in collaborations with IVF pioneers Wilfried Feichtinger and Peter Kemeter in Austria.

Dutch manufacturer Philips® followed on with one of the earliest mechanical vaginal scanners in the second half of 1986. The probe was in the shape of a microphone with a roundish elongated head housing a 5MHz 13mm wobbler transducer. It could be retro-fitted onto their real-time scanner SDR 1550 which first debuted in 1985. Although they produced excellent images compared to their abdominal counterpart, mechanical endovaginal designs were not favored by many ultrasound manufacturers, partly because of the vibration that was generated.

Mechanical designs were rapidly followed by electronic array versions which are rather like a reduced-size abdominal convex sector transducer that has appeared around the same time from other manufacturers in Japan. ALOKA® produced an electronic sector version which could be retofitted onto their older model the SSD-256. GE Medical Systems® produced their first endovaginal probe to fit their RT3200 in mid 1987. By 1988 most manufacturers had endovaginal options installed in their scanners.

Ultrasound
scanner technology continued to develop and improve in the 1980s. Real-time
scanners had rather standard appearance, sizes and fabrication. They are
usually portable on 4 wheels with the monitor on the top of the console and
rows of receptacles at the bottom to accomodate a variety of scanner probes.
See some of these scanners here. By the mid 1980s curvilinear or convex abdominal transducers have come into the market which have a better fit to the Obstetric abdomen and have a wider field of view further from the transducer face. Curvilinear arrays have completely replaced the linear configuration by the late 1980s.

Prior to the 1990s, B-scan ultrasound images made steady progress in
resolution and quality, but the improvements were not dramatic and except for
a few really top-end brands, most had felt that images in the late 1980s did
not have significant improvements over those in the early 80s. During this
period, techiques for resolution and overall image enhancement centered
around:

the increase in the number of transducer crystals (or channels, from 64
to 128), improvements in transducer crystal technology (going
into broad-band and high dynamic range), increasing array aperture (more
crystals firing in a single time-frame (with faster computational
capabilities), improving technical agorithms for focusing on receive
(increasing the number of focal zones along the beam), incoporating automatic
time-gain controls and progressively replacing analog portions of the signal
path to digital. See a brief discussion on the linear and phased-array principles.

Acuson Coporation®, a
company founded in California in 1979, marketed their first model Acuson 128 System in 1983,
employing a 128-channel "Computed Sonography platform" based on a
software-controlled image formation process. The machine shook the ultrasound
community with its excellent resolution and clarity (and also the price). Many
other companies followed on similar system designs. Other innovative
breakthroughs were seen in designs from companies such as ATL® (Advanced Technology Laboratories),
GE®
(General Electric) and Toshiba® . The early to mid- 1980s was the time with the heaviest
proliferation of standard-setting good quality machines. By the early 1980s
there were over 45 large
and small diagnostic ultrasound equipment manufacturers worldwide.

Image quality saw real improvements in the early 1990s. It is interesting
to note that the availability of new and effective technologies to ultrasound
scanners had also progressively stemmed from advances in technology in other
areas of science such as radar navigation, telecommunications
and consumer electronics. Such included the rapid developments in
cellular telephones, micro-computers, digital compact and
versatile disk players, and high definition TVs. The very
high-speed digital electronics required for ultrasound application had become
available at an affordable costs. The ultrasound imaging market alone would
not have supported the development of these new technologies.

(Model number of scanners made after 1980 from important manufacturers are
listed here with
the year in which they were marketed).

The new developments in the 1990s which has lead to some real enhancement in image quality and resolution include:

Previously the beamformer (employing analog delay lines)
and the signal processing stages are usually analog in their operation. The
digital change-over was based on the very powerful computer platforms
that were only available after the mid 1990s. The processor in the newer
highend machines has the power equivalent of roughly 40 Pentium
processors, executing some 20 to 30 billion operations per
second. Most of the processing are also programmable software-based
rather than hardware-based and allow for much more versatility and finer
adjustments in the manipulation of beam signals. Signals from and to the
transducer elements are digitized before any signal processing, which is one
of the most important advancement in ultrasound technology in the 90s. It
opened the venue for dealing with some of more difficult areas in ultrasound
physics.Superfast digital beamformers allow for many times the number of
focal points along the beam and produce microfine focal points on receive to
the size of a screen pixel. Digital beamforming also reduces noise in the
signal processing by several hundred folds producing a much cleaner picture.

2. Extensive use of refined broad-band wide aperture
transducers, improving both definition of tissue textures and
dynamic range. With wide aperture transducers, transmit and receive
apodization also allowed for the electronic reduction of the lateral array
elements (sidelobes). In the early 1990s there was much improvements in
transducer material design and fabrication technology allowing for higher
frequency transducers, improved sensitivity and contrast resolution. The
number of channels in high-end systems went up to 256 and more
recently to 512 and 1024 (2-D arrays) in several high-end
systems allowing for extremely wide aperture on transmission and reception.
In ultrasound physics, the lateral resolution is the product of the
wavelength and the f-number. The f-number equals the depth of the
returning echo divided by the aperture of the beam. (the aperture of
the beam is the width of the number of simultaneous firing transducer
elements in the array, that means the larger the aperture the more elements
are fired simultaneously). Therefore lateral resolution will be best
(smallest) if there is a large aperture and short wavelength (higher
frequency).

Too large an aperture will slow the frame rate considerably and requires
very fast computation and parallel processing. This has been made possible
with the more recent digital electronics and the very powerful
super-processors (see above). Many slightly older ultrasound systems are
capable of using low f-numbers on reception at an affordable cost. However,
they often employed large f-numbers on transmit in order to cover a large
area. Significant improvement in lateral resolution requires low f-numbers
both on transmit and receive. With the new 'very wide' aperture beamformer
(often up to 128 channels), the transmit and receive f-numbers are lowered.
The resulting improvements in lateral resolution can be as much as 4 times.

3. The phase data in returning ultrasound
echoes, in addition to the amplitude data are processed in what is
known as coherent image processing. The technique produced twice the amount
of data from which to create ultrasound images of high resolution. The frame
rate is also increased. The late 1990s has also seen transducer developing
into 2D arrays which is made up of large number of elements arranged
in rows and columns across the face of the transducer. Focusing occurs in
two directions which produced a finer and clearer definition in both planes
eliminating artifacts from adjacent tissue planes which may produce the
partial volume effect.

4. The advent of tissue harmonic imaging. The
technology, which has emerged as a major imaging trend in the last 4 years
of the 1990s, made used of the generation of harmonic frequencies as an
ultrasound wave propagates through tissue, dramatically reducing near field
and side lobe artifacts.
In a nutshell, tissue harmonic Imaging made use of lower frequency sound waves to improve penetration, while receiving and processing only the higher frequency echoes produced by the body's inherent harmonic characteristics. This process can reduce clutter and improve image clarity significantly.
As ultrasound waves propagate through tissue, there
is non-linearities in sound propagation that gradually change the shape of
the wave, a shape change that can only result from the development of
harmonic frequencies within the wave. There are no harmonic frequencies
present at the transducer face. They develop gradually as the wave
propagates through tissue, and so in the near field there is very little
harmonic energy available for reflection from tissue. Since the near field
is a source of much of the artifact in the ultrasound image, selective
display of harmonic energy will show dramatically less near-field
artifact. The strength of the harmonic energy generated is proportional
to the square of the energy in the fundamental wave. Most of the harmonic
energy results from the strongest part of the beam, and weaker portions of
the beam (side lobes, for example) generate relatively little harmonic
energy. selective harmonic imaging will yield a dramatically cleaner
contrast between adjacent tissue structures.

The development of harmonic imaging would not have been possible until
the late 1990s as there must be excellent beam linearity on transmission and
super sensitivity and dynamic range on receive to display the harmonic
energy without an unacceptable amount of noise, as the harmonic signals are
always much less in amplitude than the original fundamental signal. There
must also be a very selective and fast digital filter within the receiver,
to exclude the large percentage of the fundamental signal. Harmonic imaging
is particularly useful in obese patients. Further refinements in
harmonic imaging techniques and cost-cutting would be expected in the next
few years.

From left to right: Changes in image
quality from 1985, 1990 to 1995 respectively.There
were improvements in spatial and contrast resolution, background noise
reduction, dynamic range, and near and far field visualization.

More significant improvements came after the mid-1990's. This
image from ATL® * demonstrating fetal spine and cord.

Ultrasound scanners came into different categories according to their performance and price. From the early 1980s, scanners have started to move into clinics and private offices and there is a trend to decentralise ultrasound services all over the world. Acceptance and demand from the lay public have also increased exponentially coupled with increased utilization by various medical specialties and sub-specialties. Standards and quality of scans became an emerging problem not seen in other areas of medical imaging, where Radiogists received the relevant training and underwent apppropriate examinations before running the service. Obstetricians were simply using the scanner probe as a torch to "look inside" the uterus. Standards varied and mis-diagnosis was not uncommon. Obstetricians and Gynecologists took on the fact that they are the more suitable persons to do the scans as compared to their radiological colleagues. Special training centers and accreditation boards were gradually set up by the health authorities in the United States, Australia, Europe and other countries.

And in both a 'Consumer pull' and 'Technology Push' situation the diagnostic application
of ultrasound in the field of Obstetrics and Gynecology continued to expand
into new horizons. In 1975, that is before the advent of real-time equipments, in the United States there were only 5 legitimate indications in obstetric sonography: wmeasurement of the biparietal diameter (and other dating purposes), wdetermining amniotic fluid volume, wdiagnosis of early pregnancy failure, wevaluation of multiple gestations andwplacental localisation. The indications have since the early 80's expanded into at least 2 dozen, including most notably the accurate evaluation of fetal growth and the diagnosis of fetal malformations.

** Courtesy of KretzTechnik®, Zipf, Austria.
*** Scottish machine, images reproduced with permission from Dr. RG Law, from his book 'Ultrasound in Clinical Obstetrics', John Wright and Sons Ltd, Bristol, 1980.
**** Image courtesy of Dr. Eric Blackwell, reproduced with permission.
*^* From "Medical Diagnostic Ultrasound: A Retrospective on its 40th Anniversary", reproduced with permission from Dr. Barry Goldberg.
Pixel focusing image courtesy of Medison ®.* Copyrighted ATL®,
reproduced with permission.
- It is not possible to include all the names who have contributed significantly to the advancement of Obstetrical and Gynecological sonography, some who may have been less well-known than the others and some who may not have published so extensively in the English language. Apologies are extended to those whose contribution has not been fully credited in this article.